As is well known, a coiled magnet, if wound with wire possessing certain characteristics, can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of 10 minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
The modern MRI system requires a high-strength uniform magnetic field in a large imaging volume, for example, a field having an inhomogeneity of a few parts per million over a spherical volume having a diameter of 40-50 cm. The signal-to-noise ratio of MRI is proportional to the field strength in the imaging region. To have a high-quality image, the field strength for MRI is usually required to be larger than 0.5 T and up to 2 or 3 T. On the other hand, the stray field produced by such a magnet must be limited to a small volume to minimize the environmental impact of a magnet of such large size. For example, since an MRI system is often installed in hospitals, which contain various electronic equipment and extraneous magnetic fields surrounding the MRI system location, the equipment must be isolated from the MRI magnetic field and the MRI system must be shielded from surrounding magnetic fields. Generally, the 5-Gauss line of a MRI magnet cannot be over about 2.5 m radially and 4.0 m axially. Active shield superconducting magnet technology was developed to meet the foregoing design goals.
A typical active shield magnet consists of two sets of superconducting coils. An inner set of coils, usually called the main magnet coils, produce a uniform magnetic field of large magnitude in a imaging volume. The conventional support structure for the main magnet coils is a circular cylindrical aluminum drum. The main coils are wound separately around stainless steel bobbins, placed in grooves machined in the drum and spaced axially along the inside of the drum. Another set of outer magnet coils, usually called bucking coils, are spaced from and surround the main coils, and are supported by a structure which is secured to the drum. The bucking coils carry currents in the direction opposite to the direction of currents being carried by the main coils so as to cancel the stray magnetic field outside the magnet. This is called active magnetic shielding.
However, in the process of energizing the magnets, or ramping the magnets to field, and in cooling the coils to superconducting temperatures, the coils are subjected to significant thermal and electromagnetic loading. As a result, actively shielded magnets pose difficult problems in terms of structural support. The principal reason for utilizing actively shielded magnets, as opposed to a passively shielded system, is that the latter would require massive amounts of magnetic material, such as iron, around the magnet, which would increase both the weight and volume of the system considerably. To minimize weight and volume through use of bucking coils, and thus realize the objectives of active magnetic shielding, it is important that the support structure for the bucking coils be relatively lightweight and yet withstand the significant magnetic and thermal loads placed upon it during energization and operation of an MRI system. It is also important that the bucking coils maintain close positional accuracy, notwithstanding the significant thermal loads during initial operation or cooldown of the MRI, and notwithstanding the electromagnetic loads generated during energization and operation. As a result, there are conflicting thermal, magnetic and mechanical considerations and factors which must be balanced and compromised to obtain an acceptable bucking coil assembly.
Thus, a conventional active shield magnet has a very complicated structure and has been expensive to build. An example of a known active shield magnet is described in U.S. Pat. No. 5,237,300. In accordance with that arrangement as shown in FIG. 1 annexed hereto, a plurality of main magnet coils 4a-4f produce a highly uniform magnetic field of large magnitude. The main coils are supported by a circular cylindrical drum 2 within machined pockets or grooves. Drum 2 is typically made of aluminum alloy. A pair of bucking coils 10a and 10b concentrically surround portions of drum 2 and main magnet coils 4a-4f. The bucking coils carry currents in a direction opposite to the direction in which currents are carried by the main magnet coils, thereby providing cancellation of magnetic fields in the region outside the MRI system. The bucking coils are supported on bucking coil support cylinders or bands 6 and 8, respectively. The supports for the bucking coils include a plurality of struts or plates 12 extending angularly outward from drum 2, or from bands 16 around the drum, to bucking coil support cylinders 6 and 8, with spacing rods 14 extending axially between the bucking coil support cylinders.